The present invention relates to the processing of color TV chrominance, and more particularly to a method and apparatus for automatically adjusting dynamic color temperature state of color TV equipment such as pick-up equipment, transmitting equipment, receiving equipment, and playing equipment.
The color-temperature process of existing color TV equipment uses a single fixation method, that is, the single fixation white color temperature reference standard is used for setting up the white balance of a color TV pick-up equipment or receiving equipment. The equipment work state at the same time is used to relatively determine the functional relation between the chrominance signal input and output in a chrominance channel circuit. The functional relation will remain unchanged when the equipment processes any white or non-white image signal. Up to now, many TV technology groups in different countries in the world are still not in agreement on the point about the sampling locus of the above single fixation color temperature reference standard. The standard white light recommended presently by CIE, i.e. the Committee of International Emittance, is D65 white light with a relative color temperature of 6504 K. This standard is used in the present color TV system of PAL and SECAM. Another standard of C white light with a relative color temperature of 6770 K is used in the NTSC system. A standard of D93 with a relative color temperature of 9300 K is used in some countries. In China, the standard white light for color TV Broadcasts is D65, but there is a stipulation that "the white color temperature to be set up in production of color TV receivers may be stipulated by the professional standard according to subjective evaluation." In practice and similar to foreign enterprises, the standard is set at D65 in some enterprises or D93 in other enterprises (hereinafter abbreviated as D65 set or D93 set). Because the D65 and D93 standards have their own advantages, there is thus the phenomenon that the standard can not be unified. In television pickup and receiving systems, a color temperature is now usually chosen to be a regular white color temperature standard, and usually D65 or D93 (5400 K for pickup equipment) is chosen. According to the condition shown by receiving equipment, none of the D65, D93 or 5400 K can give consideration to the reproducing effects of various images. They have their own qualitative range and separate inferior faulty range. When reproducing white color, D93 equipment appears quite white and very clean. When reproducing a natural landscape in which there are mainly the contents of blue and green colors, there appears to be good scenery and depth of field, plentiful stratification, a distinct picture, bright colors, and more lifelike. But when reproducing the color of skin, the skin appears dark green sometimes. When reproducing a person's features, it appears more stiff. When reproducing red color, it appears overly purple sometimes. When reproducing the color of skin, the D65 equipment appears ruddy and soft. When reproducing red color, it appears bright and lifelike. But when reproducing white color, it appears over-red. When reproducing a natural landscape in which there are mainly the contents of blue and green color, it appears withered and yellow and looks like the scenery is covered with a layer of brown dust, sometimes. The "sometimes" mentioned above refers mainly to when there are not many differences between the ratios which every primitive color holds. The above conclusion can be supported by the results obtained in the first Chinese meeting comparing and assessing the quality of color TV receivers in 1987. During the comparing and assessing, its published evaluation report points out:
"It is satisfactory that D65 white color-temperature reproduces the color of skin. If the white color temperature has some deviation, an obvious difference in the colors of skin will occur so that the adjusting of white color temperature has an important bearing on receiving a color picture."
"Given the same brightness, 9300 K white color temperature has a more bright sense than 6500 K white color temperature, so that when receiving landscape, sample equipment of 9300 K color-temperature had higher marks than those of 6500 K color-temperature because they appeared to provide good scenery and depth of field, distinct picture and bright color."
The technical summary of the evaluation points out again:
"When reproducing memory colors, such as the color of skin and background, D65 white color temperature is beyond comparison with the other color temperature, but when reproducing the picture in which there are mainly the contents of blue and green color, it appears to have a less bright-colored level than higher color-temperature . . . (relative to D65) if the color temperature is much higher, the result reproducing the color of skin will be worse."
The main reasons for the above statements are as follows:
1. For the same image signal, the ratios of three primary colors of the reproduced image from different systems which have different reference standards of color temperature are different. The D65 receiver offers a 41.4% red component more than the D93 receiver. But the D65 receiver offers a 14.4% blue component less than the D93 receiver.
Assume an affine transformation to transfer the CIE chrominance figure from the XYZ system to the three base color system which is formally used in the technical standards in China. Suppose that the coordinates in the XYZ system for any color value (F) are (X.sub.F, Y.sub.F, Z.sub.F, and the coordinates in the RGB system for the same color value (F) are (r.sub.F, g.sub.F, b.sub.F, then the following formula of coordinate transformation results: ##EQU1##
Bringing the coordinates (0.313, 0.329, 0.358), (0.281, 0.311, 0.408) in the XYZ system for D65 and D93 into the above formula, the following relative coordinates in the RGB system result: ##EQU2##
Therefore, the reference white of D65 or D93 may be founded in the following mixed form: EQU (D65)=0.222(R)+0.387(G)+0.391(B) EQU (D93)=0.157(R)+0.386(G)+0.457(B).
From the above two formulas, we can see that, when two TV receivers display the same pictures, i.e. when their matrix circuits get the same drive voltage, their output voltages are different so that the ratio of the base color in the color appearing on the viewing screen is also different. Thus, the red component in the D65 receiver is 41.4% more than that in the D93 receiver {(0.222-0.157)/0.157}, the blue component in the D65 receiver is 14.4% less than that in the D93 receiver {(0.457-0.391))/0.457 }, and the green component in both of the D65 and D93 receivers may be considered as the same, because there is only a 0.2% difference.
2. The influence of the illumination source to the chrominance of the object is extensive, but the color temperature from different light sources disperses from 2000 K to 28000 K. The scenery objects appearing in many conditions are actually exposed under the light of the sky. And the color temperature from the sky light is usually higher than 6500 K, for example, the color temperature for the blue sky is from 10,000 K to 12,000 K, and for a blue sky with thin clouds, from 14,000 K to 22,000 K. Therefore, the D65 can not represent the chrominance of the object under the above conditions. For the same reason, it is also impossible for D93 to represent the chrominance of the object directly irradiated from some artificial light sources or sunlight.